The invention relates to fuel cells and in particular to fuel cells comprising an alternation of proton exchange membranes and bipolar plates.
Fuel cells are envisaged as electrical supply systems for motor vehicles produced on a large scale in the future, and also for a large number of applications. A fuel cell is an electrochemical device which converts chemical energy directly into electrical energy. A fuel such as molecular hydrogen or methanol is used as fuel for the fuel cell.
In the case of molecular hydrogen, the latter is oxidized and ionized on an electrode of the fuel cell and an oxidant or combustion agent is reduced on another electrode of the fuel cell. The chemical reaction produces water at the cathode, oxygen being reduced and reacting with the protons. The great advantage of the fuel cell is that it avoids discharges of atmospheric polluting compounds on the site of electricity generation.
Proton exchange membrane fuel cells, known as PEM fuel cells, operate at low temperature and exhibit particularly advantageous compactness properties. Each cell comprises an electrolytic membrane which allows only the passage of protons and not the passage of electrons. The membrane comprises an anode on a first face and a cathode on a second face, in order to form a membrane electrode assembly, known as an MEA.
At the anode, the molecular hydrogen is ionized to produce protons which pass through the membrane. The electrons produced by this reaction migrate to a flow plate and then pass through an electric circuit external to the cell in order to form an electric current. At the cathode, oxygen is reduced and reacts with the protons to form water.
The fuel cell can comprise several plates, known as bipolar plates, for example made of metal, stacked on one another. The membrane is positioned between two bipolar plates. The bipolar plates can comprise flow channels and orifices for guiding the reactants and the products toward/from the membrane, for guiding cooling liquid and for separating different compartments. The bipolar plates are also electrically conducting in order to form collectors of the electrons generated at the anode. The bipolar plates also have a mechanical role of transmitting the strains of clamping of the stack necessary for the quality of the electrical contact. Gas diffusion layers are interposed between the electrodes and the bipolar plates and are in contact with the bipolar plates.
Electron conduction is carried out through the bipolar plates, ion conduction being obtained through the membrane.
The bipolar plates continuously supply the reactive surfaces of the electrodes with reactants, as they are consumed. The distribution of the reactants at the electrodes has to be as homogeneous as possible over the whole of their surface. The bipolar plates comprise networks of flow channels which provide for the distribution of the reactants. A network of flow channels is dedicated to the anode fluid and a network of flow channels is dedicated to the cathode fluid. The networks of anode and cathode flow channels are never in communication inside the fuel cell, in order to prevent direct combustion of the fuel and the oxidant. The reaction products and the unreactive entities are discharged by entrainment by the flow as far as the outlet of the networks of distribution channels. In the majority of the architectures encountered, the bipolar plates comprise flow channels traversed by cooling liquid, making possible the discharge of the heat produced.
Three forms of circulation of the reactants in the flow channels are mainly distinguished:
serpentine channels: one or more channels run through the entire active surface in several to-and-from paths.
parallel channels: a bundle of parallel and traversing channels runs right through the active surface,
interdigital channels: a bundle of parallel and blocked channels runs right through the active surface. Each channel is blocked, either on the side of the fluid inlet or on the side of the fluid outlet. The fluid entering a channel is then forced to pass locally through the gas diffusion layer in order to join an adjacent channel and subsequently reach the fluid outlet of this adjacent channel.
The flow channels can be straight or slightly wavy.
The materials most commonly used for the bipolar plates are carbon-polymer composite and embossed metal.
The embossed metal proves to be a solution which favors the lightening and the compactness of the fuel cells. The bipolar plates then use thin metal sheets, for example made of stainless steel. The flow channels are obtained by embossing. Most frequently, use is made of a first flow plate in the form of a first embossed sheet defining the anode flow channel and of a second flow plate in the form of a second embossed sheet defining the cathode flow channel. These two sheets of the flow plates are assembled back to back by welding to form a bipolar plate. A flow channel for the cooling fluid is put into the space between the sheets.
The carbon-polymer composite technology makes possible greater flexibility in design of the flow channels by molding thicker plates.
The document FR 2 973 583 provides for the deletion of the cooling liquid channel in one bipolar plate out of two. With bipolar plates made of sheet metal, the bipolar plate devoid of a cooling liquid channel comprises just one embossed sheet, which lightens the fuel cell. The cathode flow channels are formed on a first face of the sheet, while the anode flow channels are formed on the other face of the sheet.
The design of these channels is then closely related, since the anode face is the negative of the cathode face. Nevertheless, the pattern of the channels has to guarantee that the flows in the bipolar plates having just one sheet are similar to those in the bipolar plates having two sheets, in order not to create an imbalance in supply between the different cells. Furthermore, it is desirable for one and the same sheet to be able to be used without distinction for a bipolar plate having a single sheet or to form a bipolar plate having two sheets.
An additional design constraint relates to the drops in pressure in the flows of reactants, these drops in pressure having to have one and the same order of magnitude at the anode and at the cathode. This constraint has an effect on the respective cross sections of the fuel and oxidant flow channels. This constraint complicates the design of a bipolar plate having just one sheet.
For molecular hydrogen used as fuel, the passage cross section in the anode channels has to be smaller than that in the cathode channels in order to obtain a drop in pressure of the same order of magnitude. This is because molecular hydrogen is less viscous than the oxidant circulating at the cathode and its flow rate is lower.
The molar flow rate of molecular hydrogen consumed in a cell is equal to I/, I being the electric current produced and F the Faraday constant. The molar flow rate of air consumed is, for its part, equal to 1.2*I/F.
In practice, the flow rates of fuel and oxidant introduced into the cells are always greater than the flow rates consumed, according to an overvaluation or mark-up factor. For molecular hydrogen, the overvaluation factor is generally between 1 and 2.5. For air, the overvaluation factor is generally between 1.2 and 3, in order to guarantee a sufficient amount of oxygen at the outlet. Thus, for a given current I, the ratio of the molar flow rate of air to the molar flow rate of molecular hydrogen is at least equal to 2 and most commonly between 3 and 5.
The viscosity of humid molecular hydrogen is of the order of 8*10−6 to 13*10−6 Pa·s, depending on the temperature and the moisture content. The viscosity of humid air is of the order of 12*10−6 to 21*10−6 Pa·s.
When the flow channels of a bipolar plate are identical on the hydrogen and air sides, a ratio of the drop in pressure for the air to the drop in pressure for the molecular hydrogen of between 2 and 10 is obtained. In order to balance the drops in pressure, it is desirable to reduce the passage cross section of the molecular hydrogen flow channels with a ratio of between 2 and 10 with respect to the passage cross section of the air flow channels.
Several alternatives are known for reducing this disproportion in drops in pressure, with associated disadvantages:                increasing the width of the cathode flow channels, producing anode channels having the thinnest width which it is industrially possible to produce. This contradicts research on the optimum effectiveness of electron conduction in the fuel cell, which is generally obtained by reducing as much as possible the width of the flow channels;        producing fewer flow channels at the anode than at the cathode. The distance between two anode channels increases, as well as the width of the teeth separating the channels. This structure is unsuitable for a bipolar plate having just one sheet as the electron conduction from the cathodes is then greatly affected;        decreasing the depth of the anode flow channels. This structure is unsuitable for a bipolar plate having just one sheet as the anode and cathode channels automatically have one and the same depth and are thus affected by this decrease in depth.        